In the Coriolis force or convective inertia force mass flowmeters, the rate of mass flow of media moving through a pair of parallel vibrating conduits under relative forced vibrations therebetween at the natural frequency is determined from the difference in the phase of the relative vibrations between two sections of the combination of the pair of vibrating conduits. At the present time, the relative vibrations between the two vibrating conduits are measured by motion detectors operating on the principles of the magnetic induction coil without any exceptions. As demonstrated by vibration sensing technology employed in the vortex shedding flowmeters, the Piezo electric vibration sensor provides far superior and greatly more sensitive motion detecting means compared with the magnetic induction coil that does not work at all in detecting the vibratory motions created by the vortex shedding phenomena. The application of a Piezo electric vibration detector to the mass flowmeter of vibrating tube types should increase the sensitivity as well as reliability of the mass flowmeter.